13 Oct
2015
13 Oct
'15
7:54 a.m.
On Oct 12, 2015, at 9:16 PM, Rich Alderson <RichA
at LivingComputerMuseum.org> wrote:
...
The M tracks are longitudinally encoded (6-bit values chosen such that they
read the same as NRZ backwards and forwards for DECtape, 4-bit values for
LINCtape) to predefine blocks (cf. disk sectors) for data.
More precisely: it's Manchester encoding, not NRZ. The result is that mark track
codes are complemented and reversed end for end if you read them in the opposite order.
The code choices are such that this process (obverse complement) produces another code
word with the right meaning for this spot of the tape in that direction. So "in the
data field of the block" reads the same in both directions. But "block
start" in one direction reads as "block end" in the other, which is just
the result you want.
The DECtape patent (3,387,293 -- on bitsavers among other places) describes this very
nicely.
paul